1 x 2 x 3 blocks. I have 6 of them
I used to be a Mold Maker.
I have about (mumble) of them... a lot, I made them all as an apprentice.
10 sets of parallels.
I also got to make 2", 4", 6", 12" and a few other specialty right angle blocks during my work.
4 matched 12" machinists vices with matched radius pivots for using wring-able gauge block.
I got to make my radius gauges, my indicator bases and a few other things.
Among other things.
Is this a secret machinist code thing?
Not particularly. A 1" by 2" by 3" block are a default and forever used part of anyone in the Machining industry. Machine operators have them and keep a fairly nice set to work on their machines they operate. Usually these are very accurately hardened and ground to within 1/2 ten-thousandth of an inch (At least mine were back in the 90s, probably off now with seasoning/relaxation of the metals)
A set of Parallels is a couple pieces of steel that are hardened and then ground down to be a matched set... to ensure things sitting flat or parallel to the mounted surface or machine deck. My sets of parallels are usually sets of 4. Though my 12x2 set is a pair. Usually parallels are also ground with within a 1/2 ten thousandth of each other.
Right Angle blocks are used to hold pieces of metal at right angles to the deck/machine table to allow machining of an opposing end, and do it squarely (90 degrees exactly)... these are really rather forced to be exactly 90 degree and 90 degrees two ways to allow usage in holding with a fence or flipping.
Machinist vices in matched sets, that are made exactly the same and ground the same and hardened in the same process and then finished assembled and re-ground... with a built in matching 12" sine plate... are very useful. Especially with LONG pieces that have to be held at the same angle over those lengths. These are near invaluable.
Radius gauges are just that... to help you get a radius (like a 1/4" radius) correctly machines or ground. My set goes from 1/16" (1/8" hole) to 2" radius (or a 4" hole).
I built many indicator bases for Dial indicators (some long travel 4" some .1"
, some bases held by a machine's "chuck" others camped onto a machine to make sure a table it tracking right or to ensure you only go a specified distance... or for aligning a piece of metal/wood/plastic/modelling putty properly.
I also made various X-blocks (quad sided/sized V-Blocks ground to match the others) and other useful tools, like a full drill set "drill index" from a 80 gauge through 1", a couple of pin-vises, tap handles, die handles, thin scribes, etc... that many people just buy now a day.